The present invention relates generally to slotted array antennas, and more particularly, to a method of using planar near field data of a slotted antenna to predict its far field pattern at extreme angles of coverage, beyond the traditional valid angle limit.
As for prior art far field pattern prediction methods for use with slotted array antennas, depending upon the size of the test scan, the size of the antenna, the distance of the probe from the antenna and the operating frequency, a planar near field test will only be able to predict the resulted far field pattern within a limited angle of coverage. In particular, there is no known prior art relating to a method of predicting the far field pattern of a slotted planar array antenna at extreme angles using planar near field data.
Accordingly, it is an objective of the present invention to provide for a method of predicting the far field pattern of a slotted planar array antenna at extreme angles. It is a further objective of the present invention to provide for a method of predicting the far field pattern of a slotted planar array antenna at extreme angles using planar near field data.
To accomplish the above and other objectives, the present invention provides for a method of predicting the far field pattern of a slotted planar array antenna at extreme angles using planar near field data. The present invention first takes the planar near field data and back-transforms it to the aperture of the antenna using widely accepted National Institute of Standards and Technology (N.I.S.T.) codes to provide X position, Y position, amplitude and phase data for the array antenna. Then, an interpolation process is used to calculate the excitation coefficient (phase and amplitude) of each slot element of the antenna using the results obtained from the back-transform. The excitation coefficients are then used as an input to an array factor routine to predict the far field pattern of the tested antenna.
The present invention enables a user or designer to utilize a planar near field data of a slotted antenna to predict its far field pattern at extreme angle of coverage, beyond the traditional valid angle limit. Depending upon the size of the test scan, the size of the antenna, the distance of the probe from the antenna and the operating frequency, a conventional planar near field test will only be able to predict the resulted far field pattern within a limited angle of coverage. The present invention is designed to overcome this limitation.
The present invention has the advantage of overcoming the planar near field limitation of validity angle of coverage. However, the ultimate advantage that motivated the present invention is test speed. In a production environment, repeated testing is necessary to effectively diagnose and ultimately correct for test failures. This process can be very time consuming if test time is significantly lengthy. The test time for a planar near field is relatively ten times faster than that of a spherical near field. A typical test time for a planar near field test is about five minutes, compared to the typical test time of a spherical near field for the same antenna of about fifty minutes. Given this fact, one would like to use planar near field data to test and diagnose antenna test failures instead of using the much slower spherical near field testing technique.
As stated above however, the conventional planar near field test has a major limitation in that it cannot predict the far field pattern of an antenna at extreme angle of coverage. Thus if the antenna has a failure at extreme angle in the far field, the conventional planar near field cannot predict it, whereas a spherical near field test does not have this limitation. The spherical near field test however, takes ten times as long compared to a planar near field test. Therefore, to minimize test time, one would like to use the planar near field testing technique instead of the more complete, but slower spherical near field testing technique.
One might ask why spherical near field-testing be used at all since it is so time consuming? The answer is that the present invention only predicts the far field pattern results at extreme angles, and does not directly measure it. For antenna sell-off purposes, quality assurance requires that antenna performance be measured directly to verify its actual performance. Consequently, prediction is not adequate for quality assurance purposes.
Thus, the present invention enables the use of a much faster planar near field testing technique to diagnose and troubleshoot planar antennas, and then the use of a much slower but more complete spherical near field testing technique for final acceptance testing of the antenna performance.
The present invention may be use to test planar antennas including those used on the F18, F15, AV8B, F14D, AMRAAM, Phalanx, and Standard missile antennas manufactured the assignee of the present invention, along with any other planar slotted array antenna.